Stand on a bridge and look down. The middle of the river is moving; along one edge, behind a sandbank, the water just sits there. Water gets in and cannot easily get out, so leaves and mud collect and the surface stays flat. That held-back water is the picture the whole word grows from — and 'held back' is also what you hear when someone calls a town a backwater, or when a rower pushes the water back to stop the boat.
可以想成你站在橋上往下看。河中間的水在動，靠岸邊、沙洲後面那一塊卻停在那裡。水進得去，出不太來，所以葉子和泥都積在那裡，水面平平的。這塊「被擋住的水」就是整個字的來源。後來說一個小鎮是 backwater，說的也是同一件事：主流過去了，它被留在旁邊。連划船時把水往回推、讓船停住的動作，用的也是這個「往回擋」的畫面。
From the word’s history
Simply back plus water: water pushed or held back.
就是 back（往回）加 water（水）：被擋回來、或被推回去的水。這個組合直接就是核心畫面，不需要繞路。
Wiktionary entries 'backwater' (noun and verb) in the supplied packet, source_refs kaikki:3f6f69a190036ae4:804765049 and :804802713: 'From back + water.'
登入就可以看每一支怎麼連回核心畫面。